Running a marine or reef aquarium is an incredibly rewarding hobby, but it comes with challenges. One of the most common struggles reef-keepers face is the introduction and spread of pests. These organisms, whether algae, invertebrates, or bacteria, can stress or even kill corals, outcompete beneficial organisms, and destabilize your aquarium ecosystem.

Below is a detailed guide to the most common marine aquarium pests, how to identify them, what causes them, and the proven methods to control or eliminate them.

1. Aiptasia (Glass Anemones)

What It Is: Aiptasia are fast-spreading nuisance anemones that can sting corals, fish, and invertebrates. They reproduce both sexually and asexually, making them hard to eradicate once established.

Causes: Usually introduced as hitchhikers on live rock, coral frags, or even in sump systems.

Risks: Can overrun tanks, sting and kill corals, and compete for nutrients and space.

Remedies:

• Manual/chemical control: Use products such as Aiptasia-X, Joe’s Juice, or kalkwasser paste injected directly into the mouth. Careful precision is needed to avoid triggering them to release planula larvae.

• Biological control: Introduce peppermint shrimp (Lysmata wurdemanni), Berghia nudibranchs (highly effective but specialized eaters), or certain butterflyfish (e.g., Copperband Butterflyfish).

• Prevention: Dip and inspect new coral frags. Remove or quarantine suspicious rock before adding to the display tank.

2. Majano Anemones

What It Is: Similar to Aiptasia but often bubble-tipped, Majano anemones are hardy and reproduce quickly.

Causes: Hitchhike on live rock or coral frags.

Risks: While less aggressive than Aiptasia, Majano can still sting corals and spread fast.

Remedies:

• Apply kalkwasser or Aiptasia-X injections.

• Remove affected rock and treat outside the aquarium.

• Some butterflyfish may feed on them, but success varies.

3. Bristle Worms

What It Is: Segmented worms with bristles that can deliver a painful sting to aquarists.

Causes: Naturally present in live rock, sand, and healthy systems.

Risks: Most species are harmless and beneficial detritivores, but large fireworms can prey on corals or small fish.

Remedies:

• Small worms should be left alone as part of the clean-up crew.

• Large/aggressive fireworms should be removed with bristle worm traps.

• Always wear gloves when handling live rock.

4. Flatworms (Planaria & Pest Flatworms)

What It Is: Small, flat-bodied worms that reproduce rapidly. Pest types (e.g., red planaria) smother coral surfaces and block light.

Causes: High nutrients, poor quarantine practices, and introduction on corals.

Risks: Coral tissue damage, light blocking, and toxins released when they die.

Remedies:

• Biological control: Introduce wrasses (Six-Line Wrasse, Melanurus Wrasse) or mandarins.

• Chemical control: Flatworm Exit or similar products, but siphon out as many as possible beforehand to avoid toxin release.

• Nutrient management: Reduce dissolved organics with skimming and water changes.

5. Coral-Eating Nudibranchs

What It Is: Camouflaged nudibranchs that specialize in eating specific coral species (e.g., Zoanthid-eating nudibranchs).

Causes: Almost always introduced on coral frags.

Risks: Can devastate coral colonies by eating tissue and laying eggs.

Remedies:

• Dip corals in iodine or coral dips (e.g., Revive, Coral RX) before introducing them to your display tank.

• Manually remove nudibranchs and their egg masses.

• Quarantine corals for at least 2–4 weeks.

6. Vermetid Snails

What It Is: Sessile snails that build calcareous tubes and release mucous webs to capture food.

Causes: Hitchhike in live rock and coral skeletons.

Risks: The mucous nets can irritate corals, causing tissue retraction and reduced growth.

Remedies:

• Break or glue over snail tubes with epoxy.

• Natural predators include bumblebee snails.

• Manual removal is often the most effective.

7. Bubble Algae (Valonia & Ventricaria)

What It Is: Algae that form shiny green bubbles attached to rock and corals.

Causes: Nutrient-rich environments, often introduced on live rock.

Risks: Rapid spread; bubbles can smother coral tissue. If popped, they release spores that spread further.

Remedies:

• Remove bubbles carefully without bursting them.

• Emerald crabs (Mithraculus sculptus) are efficient natural predators.

• Maintain low nutrients through regular water changes, skimming, and phosphate control media.

8. Bryopsis Algae

What It Is: Filamentous green algae with feather-like appearance.

Causes: High nutrient levels, imbalanced trace elements, weak water flow.

Risks: Difficult to remove; roots into rock pores, making manual removal ineffective.

Remedies:

• Raise magnesium levels (Kent Tech M has shown specific effectiveness).

• Manual removal combined with nutrient reduction.

• Improve water flow and ensure balanced nitrates and phosphates.

9. Dinoflagellates (Dinos)

What It Is: Microscopic protists that can produce toxic films and stringy mats with air bubbles.

Causes: Nutrient imbalance, often ultra-low nitrate and phosphate.

Risks: Highly toxic; can kill fish, corals, and invertebrates.

Remedies:

• Nutrient balancing: Slightly raise nitrate and phosphate levels to outcompete dinos with beneficial algae.

• UV sterilizer: Effective against free-swimming stages.

• Reduced lighting: Temporary blackouts can help.

• Maintain strong mechanical filtration and skimming.

10. Cyanobacteria (Red Slime Algae)

What It Is: Photosynthetic bacteria forming slimy red, purple, or green mats.

Causes: Excess nutrients, poor water flow, and organics buildup.

Risks: Smothers corals, lowers oxygen at night, and destabilizes reef systems.

Remedies:

• Increase water circulation.

• Siphon mats during water changes.

• Use Chemiclean (erythromycin-based) cautiously.

• Improve nutrient export with skimming, refugium, or phosphate media.

11. Green Hair Algae & Derivatives (Staghorn, Thread, String Algae)

What It Is: Fast-growing filamentous algae resembling hair strands.

Causes: Excess nutrients (especially iron), long light periods, or imbalanced dosing.

Risks: Overgrows corals and rock, shading out beneficial organisms.

Remedies:

• Manual removal with toothbrush.

• Biological control: Siamese algae eaters, tangs, foxfaces, and some blennies.

• Improve nutrient balance; avoid overfeeding and overdosing trace elements.

Preventing Marine Pests in Your Reef Tank

• Quarantine: Always quarantine new fish, corals, and invertebrates before adding them to the display tank.

• Coral Dips: Dip frags in iodine-based or commercial dips to kill hitchhikers.

• Strong Filtration: Maintain efficient nutrient export (protein skimmers, refugiums, media reactors).

• Regular Monitoring: Inspect corals and rocks regularly for early signs of pests.

• Stable Parameters: Balanced nutrients and strong flow help prevent outbreaks.